Abstract

The clinical benefit of targeting the MAPK pathway is demonstrated by the success of RAF and MEK inhibitors in the treatment of melanoma. However, response to such agents is short-lived due to the onset of resistance mechanisms that result in reactivation of ERK signalling. Additionally, many BRAF- and KRAS-mutant cancers exhibit de novo resistance. As the direct targeting of ERK may overcome the limitations of RAF or MEK inhibitors, there are several ERK1/2 inhibitors in development. SCH772984 (SCH, Merck-Schering) is an example of an ERK1/2 inhibitor that in addition to inhibiting ERK catalytic activity also inhibits the phosphorylation of residues within the ERK activation loop. Several ERK1/2 inhibitors have been described that inhibit ERK catalytic activity without modulating ERK phosphorylation e.g GDC-0994 (GDC, Genentech). Here we aim to characterise the biological relevance of the different modes of ERK inhibition.

We investigated the biochemical mechanism by which SCH inhibits ERK phosphorylation and demonstrated that it prevents the phosphorylation of ERK by MEK (T202/Y204), but does not affect MEK activity.

We characterised the effects of the distinct modes of ERK inhibition on ERK substrates, cellular pERK levels, pathway feedback activation and sensitivities of cell lines with different genetic backgrounds.

In contrast to BRAF-mutant cells, SCH and GDC treatment conferred pMEK induction in the KRAS-mutant cell line HCT116 (while still significantly inhibiting RSK phosphorylation). This is likely to be attributable to the feedback-driven C-RAF activation commonly associated with MAPK pathway inhibition in KRAS-mutant cells. In addition to pMEK, GDC also conferred an induction of pERK. However, SCH overcame pathway feedback and inhibited the ERK phosphorylation. The inhibition of pathway feedback conferred by SCH was associated with increased sensitivity of HCT116 cells to SCH (EC50 = 30 nM) compared to GDC (EC50 = 1.1 μM). In a cell panel screen comprising 25 BRAF-mutant and 63 KRAS-mutant cell lines, SCH and GDC had similar activity in BRAF-mutant cell lines. However, although KRAS-mutant cell lines were sensitive to SCH, they were largely resistant to GDC. These data suggest that due to their ability to overcome MAPK inhibitor-induced pathway feedback, ERK inhibitors that also inhibit the phosphorylation of ERK would be more effective in KRAS-mutant cancer than those that inhibit ERK activity alone.